(a) Field of the Invention
The present invention relates to a zoom lens system.
(b) Description of the Prior Art
In recent years, zoom lens systems are in great demand in the field of photographic lens systems and the like because they are excellent in portability, convenience in operation, etc. Especially, there is an increasing demand for zoom lens systems with ranges of field angle which include 47.degree., i.e., the field angle of standard lens systems. Furthermore, among zoom lens systems with large zoom ratios, zoom lens systems having a large zooming range in the telephoto range are especially demanded for.
To meet the above-mentioned demand, the zoom lens system disclosed in Japanese published unexamined patent application No. 127908/83 is known as a zoom lens system for photographic cameras which has the range of field angle including 47.degree. and zoom ratio exceeding 3.4. Said known zoom lens system comprises a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power and a fourth lens group having positive refractive power, and it conparatively resembles the zoom lens system according to the present invention.
In cases of zoom lens systems which comprise a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power and a fourth lens group having positive refractive power like the zoom lens system according to the present invention, the second lens group which is the only lens group having negative refractive power and which functions as the variator has very strong refractive power in most cases. Therefore, from the view point of correction of aberrations, there is a tendency that the number of lenses constituting said second lens group is made large and, consequently, the overall length of the lens system becomes long. The above-mentioned point is one of obstructing factors when it is intended to arrange a zoom lens system as a whole compactly.
The above-mentioned tendency is conspicuous in cases of zoom lens systems of such type that the second lens group is kept fixed. For the above-mentioned type of zoom lens systems, it is possible to make the optical performance stable and to simplify the structure of lens mount because the second lens group having strong refractive power does not move and, therefore, there occur very few factors such as eccentricity etc. that make the imaging performance unfavourable. However, when the second lens group is long as described in the above, the distance between the first and second lens groups becomes long in the wide position and the distance between the second and third lens groups becomes long in the teleposition. As a result, the lens system as a whole tends to become large.
Besides, when the second lens group is long, it is difficult to make the distance between the principal point of the second lens group and the principal point of the third lens group satisfactorily short, and the airspace required for moving the lens groups for the purpose of variation of the focal length tends to become large. The above-mentioned problem also prevents zoom lens systems from being arranged compactly.